cutting fluids

CUTTING FLUIDS

During metal cutting, heat is generated due to plastic deformation of metal and friction at the tool-workpiece interface. It will increase the temperature of both workpiece and tool. Hence, the hardness of the tool decreases. It leads to tool failure. Cutting fluids are used to carry away the heat produced during machining. At the same time, it reduces the friction between tool and chip. Cutting fluids, usually in the form of a liquid, are applied to the chip formation zone to improve the cutting conditions.

1. Functions of Cutting Fluids

(i) Cutting fluid cools the cutting tool and workpiece. The heat produced during machining is carried away by the fluid. It is done by supplying an adequate quantity of cutting fluid. It is necessary to cool the tool to prevent the metallurgical damage and to assist in decreasing friction at the tool-chip interface. It prevents the workpiece from excessive thermal distortion.

(ii) When the friction is decreased at the tool-chip interface, the tool life increases and the surface finish also increases. Cutting fluids lubricate the cutting tool and thus, it reduces the coefficient of friction between tool and work. This property reduces the energy or power consumption in removing metal. So, the wear on the cutting tool is reduced and hence, it increases the tool life.

(iii) It improves the surface finish as stated earlier.

(iv). It causes the chips to break up into small parts. It protects the finished surface from corrosion.

(v) It washes away the chips from the tool. It prevents the tool from fouling.

(vi) It prevents the corrosion of work and machine.

2. Properties of Cutting Fluids

A cutting fluid should have the following properties:

• It should have good lubricating properties to reduce frictional forces and to decrease the power consumption,

• It should have a high heat absorbing capacity.

• It should have a high specific heat, high heat conductivity and high film coefficient.

• It should have a high flash point.

• It should be odourless.

• It should be non-corrosive to work and tool.

• It should have low viscosity to permit free flow of liquid.

• It should be harmless to operators and the bearings.

• It should be stable so that it should not get oxidized or decomposed when left in the air.

• It should be transparent so that the cutting action of the tool may be observed. 

• It is especially desirable in precision work.

• It should not stain or leave residues on the workpiece surface.

• It should be economical to use. 

capstan lathes and single spindle automatics where free cutting brasses and steels are being machined.

(ii) Straight fatty oils or fixed oils:

These oils consist of animal, fish and vegetable oils. Some commonly used mineral oils are lard oil, olive oil, whale oil, cottonseed and linseed oil. The most important variety of straight fatty oil is lard oil. These oils are not stable and rapidly lose their lubricating properties. Lard oil is mainly used during thread cutting operations. These oils are more expensive and less available than mineral oil.

(iii) Mixed or compound oils:

It is the mixture of straight fatty and mineral oils. The film strength of fatty oils is retained even when diluted with 75% mineral oil. Therefore, they are much cheaper and more fluid than straight fatty oils. This oil has excellent lubrication and cooling properties. It is used for automatic screw machine work, heavy duty operations such as threading on capstan and turret lathes, thread milling etc.

(iv) Sulphurised oils:

It is one type of chemical additive oil. When sulphur (about 5%) is mixed with lard oil, it gives good lubricating and cooling qualities. This oil is called sulphurised oil. It is used for heavy-duty lathe work, gear cutting and thread grinding.

(v) Chlorinated oils:

It is another type of chemical additive oil. To prepare chlorinated oils, chlorine of about 3% is added to mineral oils. These oils are particularly effective in promoting anti-weld characteristics. If both chlorine and sulphur are used with mineral oil, they give the extreme pressure property to oil and are suitable for severe cutting operations on strong and tough materials such as stainless steels and nickel alloys.

The selection of cutting fluid depends on the following factors.

(i) Cutting speed

(ii) Feed rate

(iii) Depth of cut

(iv) Tool and workpiece material

(v) Velocity of cutting fluid

(vi) Expected tool life

(vii) Economical aspects

(viii) Life of cutting fluid.

4. Methods of Applying Cutting Fluids

During machining, based on the requirement and facilities available, cutting fluids, the cutting fluids are used in many ways such as

(i) drop by drop under gravity

(ii) flood under gravity

(iii) form of liquid jet

(iv) atomised form with compressed air.

(v) through centrifugal action.

5. Advantages of Cutting Fluids in Machining

(i) It extends the tool's life.

(ii) It improves the smooth finish of the product.

(iii) It increases the cutting accuracy.

(iv) It has good lubricity and thermal conductivity. So, it reduces the cutting and grinding temperature rapidly.

(v) It avoids the sticking or weldment of burrs due to heat generation during machining.

(vi) It reduces energy consumption.

(vii) It reduces friction provided by a metalworking fluid which results in decreased work of the operation (cutting, abrading or forming).

(viii) It keeps the cutting zone clean.

(ix) It acts as a better corrosion protection agent.